Solvent extraction



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ESTER INVENTOR.

Dec. 28, 1954 w, FRANCIS 2,698,276

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SOLVENT EXTRACTION Filed June 20, 1952 4 Sheets-Sheet 4 DMF RECYCLE 447J col IZECYCLE 4 17 RAFFlNATE-OIL+ SMALL (AMOUNTS 0F C02 AND DMFDIMETHYL FORMAMIDE (DMF) 4q EXTRAcroR 7 a2 LIQUID L 7 L H 7 C0 7 I l iLUBE OIL '7 1 OIL+ SMALL STOCK i AMOUNTSOFDMF CYCLE 57- RAFFINATE 01LHIGH QUALITY OIL EXTRACT- DMF+ SMALL AMOUNTS OF OIL M v 7 AND co 1 OILSMALL -DMF RECYCLE A ou M NTS OF DMF] 44 I I I H DMF RECOVERY 42 l I-DMF SMALL 46 AMOUNTS 0F EXTRACT OIL LOW QUALITY OIL mm: OIL

INVENTOR.

United States Patent Ofiice 2,698,276 Patented Dec. 28, 1954 SOLVENTEXTRACTION Alfred W. Francis, Woodbury, N. J., assignor to Socony-Vacuum Oil Company, Incorporated, a corporation of New York ApplicationJune 20, 1952, Serial No. 294,585

8 Claims. (Cl. 19613) The invention has to do with extraction withcertain selective solvents of various mixtures, and particularlyhydrocarbon mixtures such as lubricating oil stocks, to separate themixtures into fractions having different properties.

This application is a continuation-in-part of application Serial No.160,619, filed May 8, 1950, now Patent Number 2,631,966, issued March17, 1953.

Solvent extraction is commonly'used in refining lubricating oils, themost generally used solvents being nitrobenzene, furfural, phenol,sulfur dioxide, chlorex, and propane-cresylic acid. The choice has beenbased mainly on cost, solubility of oil in solvent, and ease ofrecovery. The recovery is usually by distillation; and this operationlimits (1) the boiling point of the solvent, which must be substantiallybelow that of the oil, and (2) the volume of solvent, all of which mustbe distilled before recycling the solvent. This means that thesolubility of oil in the sol vent should not be too low.

One of the above-mentioned solvents, furfural, and several othersolvents which might otherwise be considered, such as acetonitrile,nitromethane, and acetic acid,

' have a low solvent power for lubricating oil stocks, about three percent in the case of furfural at ordinary temperatures. This solubilityis increased somewhat at higher temperatures, but it is still necessaryto distill all of the furfural (several times the volume of oil) eachtime the solvent is used. The process of the present invention employsliquid carbon dioxide to increase greatly the solubility of oil in thesolvent and so to diminish the amount of furfural (or other solvent)used and also to avoid the necessity of distilling most of the solvent.

Liquid carbon dioxide has unusual miscibility relations withhydrocarbons and other substances. While liquid carbon dioxide isfavored by low cost, non-con rosiveness, non-toxicity, and ease ofrecovery from extract and raflinate phases, its use is limited by itscomplete miscibility with most gasoline hydrocarbons, and on the otherhand by its slight solubility for lubricating oils. For example, it hasbeen shown that the maximum solubility for a typical lubricating oil inliquid carbon dioxide is only 0.9 per cent at C.; and at 25 C., thesolubility is only 0.72 per cent. To one skilled in the art, it isobvious that these solubilities are too low for practical purposes.

Carbon dioxide has a critical temperature of 31.1 C. (88 F.).

This invention is predicated upon the discovery of an advantageous andpractical solvent extraction procedure in which liquid carbon dioxide isused in combination with certain solvents with which it cooperates.

I. PRIOR ART ing the removal of unsaturated hydrocarbons from. gas

mixtures.

Several investigators have also described a selective.

separation of hydrocarbons in which liquid carbon dioxide is used incombination with liquid SO; (2,034,495-4 e. g., extraction of lightgasoline; 2,346,639; German 546,123). Refining of petroleum fractions toeifect removal of sulfur compounds, using S03 in combination with liquidcarbon dioxide is disclosed by Gary (1,893,138).

Milmore (2,130,147) has described solvent extraction techniques in whicha hydrocarbon mixture containing a solvent such as chlorex, is treatedwith a low molecular substance in the para-critical state in order toprecipitate out one or more hydrocarbon fractions. One such lowmolecular substance is carbon dioxide.

Pilat and Godliewicz (2,188,013 and 2,315,131) have also extractedhydrocarbon mixtures with a conventional solvent and a gaseous treatingagent suchas a hydrocarbon or carbon dioxide. The gaseous treatingagent, such as CO2, is used in small amounts under conditions at whichit is incapable of being liquefied. Similarly, if a naphthenic solventsuch as liquid S02, furfural or the like, is used, it isv employed insmall quantity in order' to substantially saturate the oil withoutcausing the oil to separate into two phases at the treating temperature.This imposes a severe limitation upon the amount of furfural, forexample, which can be used; generally, only about two percent by weightof furfural can be used under such conditions.

Low molecular weight parafiins, such as propane or isopentane, have alsobeen used with liquid carbon dioxide in extracting hydrocarbon mixtures.This is shown by Lantz (2,188,051) and by Webb (2,246,227). Theprocesses described by these patentees are essentially precipitationprocesses, rather than simple. extractions. Lantz points out: It will benoted that as progressively larger quantities of liquid carbon dioxideare used, the amount dissolved therein decreases. Webb makes a verysimilar statement.

Typical of the diluents shown by Webb (2,246,227) for use with liquidcarbon dioxide are: hydrocarbons such as propane,- solvents such asacetone, pyridine, liquid S02, cresylic acid and other solvents of thetype of preferential solvents for non-paraflinic hydrocarbons. A similardisclosure is found in 2,281,865 (Van Dijck); used with liquid carbondioxide are hydrocarbons and solvents known as selective or naphthenic(or aromatic) solvents such as furfural, nitrobenzene, chlorex, cresylicacid, phenol, aniline and quinoline. The solvents used by Van Dijck arenecessarily employed in relatively small quantities, actually only thosequantities which are miscible with an oil. In the cases of furfural,aniline and phenol, the quantities used are minute-of the order of oneor two per cent. One salient feature of the Van Dijck process is changein solubility relationships affected by pressure changes, with pressuresused being sufficiently high, as 1200-1800 p. s. i., to preclude theformation ofa gas phase.

II. OUTLINE OF INVENTION It has now been discovered that solventextraction refining procedures in which certain solvents (S) are used,or might otherwise be considered suitable, are made much moreefiicaciousby the use therein of liquid carbon dioxide in combination with suchsolvents. In the process of this invention, liquid carbon dioxide isused in such quantities and with such solventsas to increase greatly thesolvent power of the solvent for the hydrocarbon fractions which aretreated therewith.

The solvents (S) with which liquid carbon di xide is used, arecharacterized by: (1) complete miscibility with liquid carbon dioxide,and (2) a low solvent power for components of the mixture treated, suchas a lubricating oil stock--generally, less than about five per cent (bvweight) of such mixture will dissolve in the solvent.

The sequence of operations used in effecting separation of. the severalfractions includes:

(a) Contacting lubricating oil stock with li uid car- (c) Removingcarbon dioxide from the extract phase, whereupon two additional phasesare formed upon setbefore recycling.

III. OBJECTS It is an object of this invention, therefore, to provide aneffective means for separating a multiplicity of fractions or compoundsof different properties from mixtures containing the same.

It is also an object of this invention to provide for the selectiveseparation of several hydrocarbon fractions of different properties fromhydrocarbon mixtures. An important object is the selective separation ofseveral hydrocarbon fractions, differing in properties, from highmolecular weight hydrocarbon mixtures, and particularly from lubricatingoil stocks. A further object is to selectively separate polycyclicaromatic hydrocarbons from mixtures containing the same. Still anotherobject is to selectively separate alkyl benzenes from mixturescontaining the same.

An important object of the invention is the improvement ofsolventextraction processes in which solvents of type (S) are used, byincreasing substantially the solubility of an oil therein, by decreasingappreciably the amount of such solvent required for the extraction andby reducing or obviating the necessity for recovering such solvent bydistillation.

Other objects and advantages of the invention will be apparent from thefollowing description.

IV. INVENTION IN DETAIL As indicated above, liquid carbon dioxide and asecond solvent are used together in the selective separation process ofthis invention. Solvents used with liquid carbon dioxide and designatedby the symbol (S), are miscible with liquid carbon dioxide andincompletely miscible with the mixture to be extracted. In addition,solvents in this group do not form solid salts with carbon dioxide attemperatures of 20 and greater. Preferably, too, if the solvent are notnormally liquid, they should become so in contact with liquid carbondioxide. This generally occurs if their melting points are less thanabout 60 C.

The solvent power of the solvent (S) is increased substantially bv theaddition to one part by weight thereof of amounts of liquid carbondioxide varying from about 0.25 part (weight) to about 5 parts (weight),according to the particular solvent (S) used. Thus, the present processis distinctively different from earlier processes in which liquid carbondioxide has been used with certain of the same co-solvents (S), such assulfur dioxide. For example. Andrews and Fenske (2,346,639) havedescribed their process with the following statements:

The solvents employed usually contain two ingredients, one of which is abetter solvent for the hydrocarbons than the other so that thedissolving capacity of the mixture can be varied by changing the ratioof the ingredients. This range may be extended in the direction of lowerdissolving capacity by mixing with the methylamine (or sulfur dioxide) aliquid, miscible therewith, which has a lower capacity for dissolvinghydrocarbons. These modifying solvents are chemically inert, are solublein them. and possess a lower dissolving capacity for a particularhydrocarbon type.

It is evident that Andrews and Fenske, and others. have contemplatedintermediate solvent power for mixtures of two solvents. Tn no sensehave they contemplated the possibility that mixtures of any two solventsmight have much greater solvent power than either one alone, as I havediscovered with liquid carbon dioxide and the cosolvents (S) describedherein. This solvent power relationship is particularly remarkableinasmuch as the two solvents are mutually-inert chemically.

Included among the above-described class of solvents (S). are thefollowing: furfural, triacetin, ethyl maleate, beta-chloroethylacetate,acetonitrile, ethanol, methanol, methyl formate, dimethylformamide,beta-methoxy ethanol (methyl Cellosolve), chloroacetone, ethyl sulfate,nitroethane, sulfur dioxide, acetone, acetic acid, acetic anhydride,acetonylacetone, ethylene diformate, methyl sulfate and nitromethane.

The foregoing solvents and liquid carbon dioxide are effective inresolving various mixtures into a multiplicity of fractions, orcompounds, of different properties. They are particularly advantageousin the resolution of lubricating oil stocks, representative of which arePennsylvania, Mid-Continent and Coastal types, or parafiinic, naphthenicand aromatic types. Other hydrocarbon mixtures successfully treatedinclude gas oils, fuel oils, shale oils, transformer oils, cable oils,coal tar fractions, etc. In general, therefore, hydrocarbon mixturestreated as described herein range from gas oils through lubricating oilstocks. The hydrocarbon mixtures are generally within the molecularweight range of about 150 to about 500.

The invention is illustrated by experimental data obtained with liquidcarbon dioxide and solvents of type (S). These experimental data arepresented in the form of charts, or more particularly ternary diagrams,identitied here as Figures 1 to 7. These diagrams can be used todetermine: the suitability of a solvent for a desired separation; theapproximate selectivity of the solvents; the range of compositions ofliquid carbon dioxide, solvent (S), and the mixture to be treated; andapproximate number of stages or extractions necessary to effect aseparation of desired degree; etc.

Figure 1 represents the system liquid carbon dioxide, furfural andlubricating oil. It is to be understood that in Figure 1 and in all ofthe ternary diagrams involving lubricating oil, the phase boundaries arenecessarily ap-' proximate since the oil is not a pure substance. infact, the purpose of the fractionations is to separate it into fractionsof different properties, which include miscibilit-ies with 'thesolvents. Therefore, with countcrcurrent operation in extractor 3 ofFigure 8 (described hereinafter), for example, the effective diagram ofFigure 1 is appreciably different at the bottom and at the top of theextractor. At the top of the extractor, curve fie CO2 of Figure 1 isfarther from the left side indicating a greater solubility. At thebottom of extractor 3, the reverse change appears. No substantialdifference in temperature or pressure indifferent sections of theextractor is necessary.

The oil is a highly naphthenic distillate stock having the followingproperties:

A. P. .I. gravity 23.80 Density 0.910 Refractive index, n 1.5076Critical solution temperature (with aniline), C 72 Four point, "P 20Flash (open cup), F 395 Fire, 455 Viscosity, centistokes at F 28.65Viscosity, cent-istokes at 210 F 4.51 Viscosity gravity constant 0.871Color, Lovibond 18 Figure 1 reveals that furfural and liquid carbondioxide are completely miscible, that furfural has a low solvent powerfor the oil, and that liquid carbon dioxide has a low solvent power forthe oil. The solubility of oil in furfural is three per cent attemperatures of the order of 20-25 C., as indicated by point ofFigure 1. This solubility is increased by the addition of liquid carbondioxide. From three per cent at f, the solubility is increased to aboutsixteen per cent at i, by using about sixty per cent by weight of liquidcarbon dioxide in the solvent mixture; similarly, the solubility isincreased to about fourteen per cent at e, by using about seventy percent of liquid carbon dioxide. A system .9 is assembled with aboutthirty per cent oil, twenty per cent furfural and fifty per cent carbondioxide. This separates into two layers e (upper) an extract layer and r(lower) a raffinate layer.

Following separation of layers e and r and release of carbon dioxide (toE and R, respectively), about eighty per cent of the oil dissolved inthe extract layer e is separated (at the top), leaving a dilute furfuralsolution f which is recycled without distillation. The only furfuralwhich need be recovered by distillation or by other means is. therelatively small amount dissolved in the rafiinate R, and the smallamount dissolved in the second oil layer (the extract-raflinate); abouttwo per cent of furfural is present in each of such layers. Thisrepresents a saving of about ninety per cent in distillationrequirements over conventional furfural extractions.

The line i-p, represented by dashes, is an isopycnic or line connectingcompositions i and p of liquid phases in equilibrium having equaldensities. Such a line, shown also by dashes in Figures 2, 4, and 7(described hereinafter), occurs on these diagrams when the solvent isheavier than the oil, since carbon dioxide is lighter than the oil. Theisopycnic may be at the kink of the curve, as in Figures 1 and 7, orelsewhere. It is of practical importance since an attempt at solventextraction using a system of composition on an isopycnic would beprevented by failure of the phases to settle into layers. It should benoted that if a smaller proportion of liquid carbon dioxide were used,the extract would be on the curve ;fi and the raffinate would be on thecurve pr, of Figure 1. In such case, the extra-ct layer is heavier thanthe rafiinate layer; and the flow sheet for such an operation would besimilar to that of Figure 9 rather than that of Figure 8.

A ternary diagram similar to Figure 1 is obtained for the system: liquidcarbon dioxide -triacetin -o-il.

Figures 2 through 7, inclusive, are ternary diagrams of lubricating oilstocks and liquid carbon dioxide and other solvents of type (S) used inplace of furfural.

Figure 2 shows the ternary system of liquid carbon dioxide andlubricating oil with ethyl maleate. A similar diagram is obtained whenbeta-chloroethyl acetate is used in place of ethyl maleate.

Figure 3 represents the systems formed by liquid carbon dioxide andlubricating oil, with such solvents as:

Acetonitrile Ethanol Methanol Methyl formate Figure 4 represents thesystems formed by liquid carbon dioxide and lubricating oil, and asolvent from the following group:

Dimethylformamide Beta-methoxyethanol Figure 5 represents the systemsformed by liquid carbon dioxide and lubricating oil, and a solvent suchas:

Chloroacetone Ethyl sulfate Nitroethane Sulfur dioxide 1Figure 6: liquidcarbon dioxideacetone-lubricating o1 Figure 7 represents the systemsformed by liquid carbon dioxide and lubricating oil with one of thefollowing solvents:

Acetic acid Acetic anhydride Acetonylacetone Ethylene diformate Methylsulfate Nitromethane The data from which Figures 1-7 were prepared, wereobtained with a visual autoclave, operating at room temperature, about25 C. The autoclave is a Jerguson gauge of 116 parts by volume capacitwith thick narrow Pyrex glass windows front and back. Incandescent lampsare mounted behind the vertical position of the autoclave. Agitation ofthe materials is obtained by rotation of the autoclave, end-over-end,within a heat-insulated case. The latter is provided with strip heaterswhich permit heating by radiation, and with means for cooling to lowtemperature. The autoclave was charged with the liquid reagents, thecarbon dioxide being introduced from a cylinder. Solubility of carbondioxide in another liquid was estimated by charging a definite volume ofthat liquid and then adding carbon dioxide until after agitation a newliquid phase appeared (at the top). Then additional increments of liquidcarbon dioxide were added. By extrapolation, the drop in equilibriumposition of the interface could be used to estimate approximately thesolubility of the other liquid in liquid carbon dioxide. If there was noseparation into two liquid phases, the miscibility was considered to becomplete only after about three volumes of carbon dioxide were added forone of the other liquids.

In each case the solvent (S) was removed from the rafiinate bydistillation or by washing with water or alkali, as appropriate to thesolvent. The rafiinate oil was light colored, less dense, and moreparaffinic than the charge stock as shown by lower refractive index andhigher critical solution temperature with aniline.

Additional illustrative examples of the invention are the following:

Example 1 Ten volumes of the lubricating oil stock described above, fivevolumes of acetone and twenty-one-volumes of liquid carbon dioxide werecharged to a visual autoclave. After agitating these materials for abovefive minutes at 25 C., two layers were obtained. The lower layercomprised thirteen volumes and the upper layer comprised twenty-threevolumes. These layers were removed separately from the autoclave. Carbondioxide was removed from the extract, and the carbondioxide-free extractwas allowed to settle, whereupon an extract and an acetone phase formed.Acetone was removed from the extract and raflinate, whereby a low and ahigh quality oil were obtained, respectively.

Properties of the oil products, after removal of both solvents (carbondioxide and acetone) are as follows:

Aniline Refractive Index, ne o Oil charge (ten volumes) 1. 5076 72Rafiinate (two volumes) 1.5056 74 Extract (seven volumes) 1. 5119 68Example 2 Aniline Refractive Index, m s

Oil charge (ten volumes) 1. 5076 72 Ratfinate (nine volumes) 1. 5063 73E xtract (one volume) l. 5136 66 Example 3 The lubricating oil describedabove was treated with a solvent mixture comprising about thirty percent (weight) of furfural and about seventy per cent (weight) of liquidcarbon dioxide, in the same manner as described in Examples 1 and 2,above. The results of this treatment are indicated by the followingtabulated data:

- Aniline Refractive Index, m o

Oil charge (20 volumes). 1. 5076 72 Raffinate (13 volumes) 1. 5937 76 1.5256 In order that the invention may be more fully understood, typicalseparations are described below with reference being made to thedrawings attached hereto.

In Figure 8, a charge such as a highly naphthenic lubricating oil stock,for example, one having a density of 0.910, a refractive index (n of1.5076 and a critical solution temperature (aniline) of 72 C., in tank 1is introduced through line 2 to extractor 3. Furfural in tank 4, isintroduced through line 5 into an intermediate section of 3, and liquidcarbon dioxide in tank 6 is intro duced through line 7 to a lowersection of 3 such that each solvent flows countercurrent to portions ofthe oil charge. For example, the solvent mixture comprises about thirtyper cent of furfural and seventy per cent of liquid carbon dioxide. Itwill be understood that the extractor 3 can comprise conventionalcountercurrent stage or tower extraction equipment. Contact of furfuraland liquid carbon dioxide with the oil can also be aided by conventionalpacking material in extractor 3.

The temperature of the oil and solvents in extractor 3 should not bemuch greater than about 31.1 C., the critical temperature of carbondioxide. Slightly higher temperatures, up to about 36 C., can be used insome cases because enough solvent (S) and/or hydrocarbon may dissolve inthe carbon dioxide-rich phase to make it liquid above the criticaltemperature of pure carbon dioxide. This temperature condition can berealized by maintaining both the oil and solvents at the requiredtemperature prior to introduction to the extractor 3, or the latter canbe maintained at the required temperature by well known cooling or heatexchange means. The pressure in extractor 3 is maintained sufiicientlyhigh so as to maintain a phase rich in carbon dioxide in the liquidstate.

The ratio of solvent components, liquid carbon dioxide to furfural orother solvent (S) in the extractor 3, is such that the solvent power forthe oil is substantially greater than that of either of the individualsolvents alone. For example, with furfural as the solvent (S), the ratioof liquid carbon dioxide to furfural, or related solvent (S), isgenerally between about 2:1 and about 1:2, preferably between about 3:2and 2:3. However, with acetone as the solvent (S), the ratio of liquidcarbon dioxide to acetone, if maintained from about 1:4 to about 4:1,and preferably from about 1:3 to about 1:2. Correspondingly, the ratioof liquid carbon dioxide to solvent (S) is from about 2:1 to about 5:1,and preferably from about 3:1 to about 4:1, when the solvent (S) is oneof the following: acetic acid, acetonylacetone, ethylenediformate,

methyl sulfate, or nitro-methane. The quantitative relationships betweenthe quantities of solvents (S) and liquid CO2 recited here are shown bythe several ternary diagrams constituting Figures 1 through 7.

In the extractor 3, a rafiinate phase and an extract phase are formed.The raffinate phase (the heavier phase) contains paraffins andnaphthenes. Also present in the raflinate phase are small proportions offurfural and carbon dioxide. The raflinate phase is Withdrawn fromextractor 3 through line 8 to solvent recovery vessel 9. The rafiinateis fractionated in 9, with the furfural and carbon dioxide being takenoverhead through line 10 and recycled through vessel 6 and line 7. Thisfractionation can be accomplished by using high temperature, or it canbe accomplished by countercurrent extraction with liquid carbon dioxideas described in copending application Serial No. 162,587, filed May 8,1950, now Patent Number 2,646,387, issued July 21, 1953. A rafiinate oilfraction is removed from 9 through 11; this fraction is high qualityoil, that is, it has a substantially higher viscosity index than theoriginal charge oil and other fractions thereof.

The extract phase in extractor 3 is removed through line 12 to vessel13, which is equipped with suitable means for effecting release ofcarbon dioxide from the extract phase. For example, a heat exchangemedium can be circulated through the wall of vessel 13 or through coilstherein, to raise the temperature of the extract phase. Also, pressurereducing means can be provided. In vessel 13, then, carbon dioxide isremoved through line 14 and recycled to reservoir 6 via line 10. In line14, a condenser (not shown) is used when carbon dioxide is removed byapplying heat to vessel 13, or a compressor (not shown) is positioned inline 14 when carbon dioxide is removed by pressure reduction. Theextract phase, substantially free of carbon dioxide, is taken throughline 15 to settler 16. When the extract phase is allowed to stand insettler 16, a further separation takes place, with the formation of asolvent phase (furfural) and an extract phase. The extract contains thelow quality lubrieating components of the charge and some furfural. Thisis taken through line 17 and fractionated in solvent recovery vessel 18.Again, the fractionation can be accomplished by distillation or bycountercurrent extraction with liquid carbon dioxide. Furfural andcarbon dioxide (if used) are taken overhead through line 19 and recycledto reservoir 4. The low quality oil is removed from 18 as a bottomproduct through line 20. The oil removed through line 20 has asubstantially lower viscosity index than the original charge and otherfractions thererecovery vessel 38.

of, and is suitable for use, for example, as an insecticidal oil.

The solvent phase (furfural) separated in settler 16 is recycled throughline 21 to an intermediate section of extractor 3. There is no need todistill furfural and any residual carbon dioxide present in the solventphase.

Figure 9 represents another typical separation, but one in which one ormore of the phase relationships differ from those shown in Figure 8,above. The solvent (S) used in the separation illustrated by Figure 9 isdimethyl formamide (DMF); liquid CO2 is the other solvent and the chargeoil is the same as described in connection with Figure 8.

The charge oil in vessel 30 is introduced through line 31 to extractor32. Liquid CO2 in tank 33 is introduced through line 34 to anintermediate section of 32, and DMF in tank 35 is brought through line36 to an upper section of 32. Here too, each of the solvents preferablyflows countercurrent to portions of the oil charge.

A rallinate phase and an extract phase are formed in extractor 32. Theextract phase is the heavier phase and comprises DMF and relativelysmall amounts of C02 and low quality components of the charge oil. Theextract phase is taken from extractor 32 through line 37 to C02 The.extract is fractionated in 33, such that CO2 is taken overhead therefromthrough line 39 and is recycled through vessel 33 and line 34 toextractor 32. The extract phase, now substantially free of CO2, isremoved from 38 through line 40 to settler 41. Upon standing in settler41, the extract phase separates into two layers. A solvent phaseDMF-andan extract phase are formed. The extract contains the low qualitylubricating components of the charge oil and some DMF. This extract istaken through line 42 to solvent recovery vessel 43, wherein DMF isremoved as an overhead product through line 44 and is recycledtherethrough to extractor 32 via 35 and 36. Low quality oil is removedas a bottoms product from vessel 43 through line 45.

The solvent phaseDMF-in settler 41 is recycled through line 46 to anintermediate section of extractor 32. Again, there is no necessity fordistilling DMF and any residual CO2 therefrom.

The raflinate phase in extractor 32 is the lighter phase. This isremoved through line 47 to CO2 recovery vessel 48. The raflinate phaseis fractionated in 48, with CO2 being removed overhead through line 49and being recycled via 3933--34 to extractor 32. The substantiallyCOz-free rafiinate, comprising oil and a relatively small amount of DMF,is removed from vessel 48 through line 50 to DMF recovery vessel 51. DMFis fractionated overhead through line 52 and is recycled through 4435-36 to extractor 32. Raffiuate oil is removed from vessel 51 throughline 53; this oil is high quality oil--of appreciably higher viscosityindex than the original oil charge.

It will be recognized that the foregoing illustrations provided byFigures 8 and 9 are diagrammatic, and that pumps, heaters, coolers, heatexchangers, pressure vessels of various character can be employed.

As indicated above, carbon dioxide is used in liquid form, thusrequiring the use of relatively low temperature and hlgh pressures. Ineffect, the operating temperatures will be not more than slightly abovethe critical temperature of carbon dioxide, namely, 31.1 C., andpreferably below it. While the temperature can be lowered considerablybelow 31.1 C. satisfactory operation has been realized with temperatureswithin the range of 10 C. to 35 C. Operating pressures are relativelyhigh, generally about 1000 pounds per square inch (or atmospheres).Usually, pressures are of the order of 600 to 1200 pounds per squareinch, depending upon the temperatures employed.

I claim:

1. The process of separating a hydrocarbon mixture selected from thegroup consisting of hydrocarbon mixtures within the range of gas oils tolubricating oil stocks, into fractions at least one of which has ahigher viscosity index than that of the original mixture, whichcomprises: contacting the mixture with liquid carbon dioxide and asolvent (S) under sufiicient pressure to maintain a carbon dioxide-richphase in the liquid phase, said solvent (S) being selected from thegroup consisting of furfural, triacetin, ethyl maleate, beta-chloroethylacetate, acetonitrile, ethanol, methanol, methyl formate, di-

methyl formamide, beta-methoxyethanol, chloroacetone, ethyl sulfate,nitroethane and sulfur dioxide, said solvent (S) being incapable ofbeing rendered miscible with said hydrocarbon mixture upon the additionthereto of liquid CO2, the ratio of the quantity of liquid carbondioxide to the quantity of solvent (S) being from about 2:1 to about 1:2by weight, whereupon a rafiinate-phase and an extract phase are formed;efiecting phase separation of the raffinate and extract under saidpressure; removing carbon dioxide and solvent (S) from the raffinatephase, thereby obtaining a hydrocarbon fraction of higher viscosityindex than that of the original hydrocarbon mixture.

2. The process as defined by claim 1 wherein the ratio of the quantityof liquid carbon dioxide to the quantity of solvent (S) is from about3:2 to about 2:3.

3. The process as defined by claim 1 wherein the hydrocarbon mixture isa lubricating oil stock.

4. The process as defined by claim 1 wherein said solvent (S) isfurfural.

5. The process as defined by claim 1 wherein the solvent (S) is liquidsulfur dioxide.

The process as defined by claim 1 wherein the solvent (S) is dimethylformamide.

7. The continuous process of separating a hydrocarbon mixture selectedfrom the group consisting of hydrocarbon mixtures within the range ofgas oils to lubricating oil stocks, into fractions at least one of whichhas a higher viscosity index than that of the original mixture, whichcomprises: contacting the mixture with liquid carbon dioxide and asolvent (S) under sufiicient pressure to maintain a carbon dioxide-richphase in the liquid phase. said solvent (S) being selected from thegroup consisting of furfural, triacetin, ethyl maleate, beta-chloroethylacetate, acetonitrile, ethanol, methanol, methyl formate, dimethylformamide, beta-methoxyethanol, chloroacetone, ethyl sulfate,nitroethane and sulfur dioxide, said solvent (S) being incapable ofbeing rendered miscible with said hydrocarbon mixture upon the additionthereto of liquid CO2, the ratio of the quantity of liquid carbondioxide to the quantity of solvent (S) being from about 2:1 to about 1:2by Weight, whereupon a raffinate phase and an extract phase are formed,eifecting phase separation of the rafiinate and extract under saidpressure; removing carbon dioxide and solvent (S) from the raifinatephase, thereby obtaining a hydrocarbon fraction of higher viscosityindex than that of the original hydrocarbon mixture; removing carbondioxide from the extract phase; settling the extract phase substantiallyfree of carbon dioxide, whereupon an extract and a solvent (S) phase areformed; effecting phase separation of said extract and said solvent (S)phase; and recycling directly to said contacting operation, said solventphase.

8. The process as defined by claim 7 wherein the solvent (S) isfurfural.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,023,375 Van Dijck Dec. 3, 1935 2,034,495 Sullivan Mar. 17,1936 2,166,140 Hansley July 18, 1939 2,188,051 Lantz Jan. 23, 19402,246,227 Webb June 17, 1941 2,281,865 Van Dijck May 5, 1942 2,346,639Andrews et a1 Apr. 18, 1944 2,631,966 Francis Mar. 17, 1953

1. THE PROCESS OF SEPARATING A HYDROCARBON MIXTURE SELECTED FROM THEGROUP CONSISTING OF HYDROCARBON MIXTURES WITHIN THE RANGE OF GAS OILS TOLUBRICATING OIL STOCKS, INTO FRACTIONS AT LEAST ONE OF WHICH HAS AHIGHER VISCOSITY INDEX THAN THAT OF THE ORIGINAL MIXTURE, WHICHCOMPRISES: CONTACTING THE MIXTURE WITH LIQUID CARBON DIOXIDE AND ASOLVENT (S) UNDER SUFFICIENT PRESSURE TO MAINTAIN A CARBON DIOXIDE-RICHPHASE IN THE LIQUID PHASE, SAID SOLVENT(S) BEING SELECTED FROM THE GROUPCONSISTING OF FURFURAL, TRIACETIN, ETHYL MELEATE, BETA-CHLOROETHYLACETATE, ACETONITRILE, ETHANOL, METHANOL, METHYL FORMATE, DIMETHYLFORMAMIDE, BETA-METHOXYETHANOL, CHLOROACETONE, ETHYL SULFATE,NITROETHANE AND SULFUR DIOXIDE, SAID SOLVENT (S) BEING INCAPABLE OFBEING RENDERED MISCIBLE WITH SAID HYDROCARBON MIXTURE UPON THE ADDITIONTHERETO OF LIQUID CO2, THE RATIO OF THE QUANTITY OF LIQUID CARBONDIOXIDE TO THE QUANTITY OF SOLVENT (S) BEING FROM ABOUT 2:1 TO ABOUT 1:2BY WEIGHT, WHEREUPON A RAFFINATE PHASE AND AN EXTRACT PHASE ARE FORMED:EFFECTING PHASE SEPARATION OF THE RAFFINATE AND EXTRACT UNDER SAIDPRESSURE; REMOVING CARBON DIOXIDE AND SOLVENT (S) FROM THE RAFFINATEPHASE, THEREBY OBTAINING A HYDROCARRBON FRACTION OF HIGHER VISCOSITYINDEX THAN THAT OF THE ORIGINAL HYDROCARBON MIXTURE.